Ultra-low temperature resistant sodium-ion battery electrolyte and preparation method thereof

Abstract

This invention relates to the field of sodium-ion battery technology, and discloses an ultra-low temperature resistant sodium-ion battery electrolyte and its preparation method. The electrolyte comprises a mixed solvent of propylene carbonate and methyl difluoroacetate, sodium difluorosulfonylimide as the main salt, sodium difluorooxalate borate and tris(trimethylsilyl) phosphite as film-forming aids, and hexamethyldisilazane as an acid-removing additive. The electrolyte contains mesoscopic contact ion pair clusters formed by the aggregation of film-forming aids in the sodium ion solvation layer. The preparation method includes: low-temperature mixing of the solvent and main salt; addition of additives for acid removal; isothermal ripening under heating and pressure to thermodynamically drive cluster assembly; and finally, rapid quenching to freeze and retain the cluster structure using kinetic hysteresis. This invention overcomes the problem of kinetic hysteresis in film formation at ultra-low temperatures, enabling the in-situ generation of a high-strength cross-linked interfacial film, significantly reducing interfacial impedance, and greatly improving the discharge capacity and overall performance of sodium-ion batteries under extremely cold conditions.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to an ultra-low temperature resistant sodium-ion battery electrolyte and its preparation method. Background Technology

[0002] Sodium-ion batteries have broad application prospects in large-scale energy storage and power batteries due to their abundant resources and cost advantages. However, the electrochemical performance of sodium-ion batteries is severely limited under ultra-low temperature conditions, such as in extremely cold regions. The main reason for this problem is that the macroscopic viscosity of the liquid phase of conventional sodium-ion battery electrolytes increases sharply under ultra-low temperature environments, resulting in a significant decrease in ionic conductivity and a sharp increase in the battery's polarization resistance.

[0003] To improve the low-temperature adaptability of batteries, the industry typically optimizes electrolyte formulations by introducing low-freezing-point solvents and adding composite film-forming materials. However, actual testing has revealed severe hysteresis in the film-forming kinetics at the electrode interface when the ambient temperature drops to the ultra-low temperature range. Monomeric film-forming materials dispersed in a free state in conventional electrolytes are limited by extremely high solvent desorption activation energies, making it difficult for them to undergo effective co-reduction reactions on the negative electrode surface. Because the film-forming materials cannot function effectively in time, the battery electrode interface is mainly covered by byproducts of organic solvent reactions. The resulting solid electrolyte interface film has a single composition and weak physical strength, making it highly susceptible to micro-crack failure under the shrinkage stress of ultra-low temperature electrodes. This poor interface condition leads to a significant increase in interfacial impedance, making sodium ion insertion / extraction extremely difficult, ultimately causing a severe decline in the battery's discharge capacity retention and median voltage plateau.

[0004] Furthermore, existing electrolyte preparation processes often fail to effectively address the aforementioned kinetic problems at the micro-interface. Conventional electrolyte preparation typically employs room-temperature mechanical mixing or simple heating and dissolution processes. At room temperature, the steric hindrance of solvent molecules prevents the film-forming precursors from effectively assembling into clusters. Even after heating and curing, if a conventional natural slow cooling process is subsequently used, the micro-coordination structure formed within the liquid phase will undergo structural relaxation and degenerate during the slow cooling phase. This degradation not only produces disordered secondary solvation coordination but also forms high-barrier configurations that are more difficult to dissociate than at room temperature, causing the film-forming material to completely lose its ability to co-reduce. Therefore, how to synergistically improve both electrolyte composition compatibility and the thermodynamic and kinetic control of the preparation process to solve the problem of sluggish film-forming kinetics at the electrode interface under extremely cold conditions is a pressing technical issue in the field of sodium-ion batteries. Summary of the Invention

[0005] The technical problem solved by this invention is that conventional sodium-ion battery electrolytes exhibit increased macroscopic viscosity and significantly decreased ionic conductivity under ultra-low temperature conditions, and the film-forming kinetics at the electrode interface become sluggish, making it difficult for free film-forming additives to undergo co-reduction reactions. The battery electrode interface is covered by by-reaction products of organic solvents, resulting in high interfacial impedance, weak physical strength, and severe decay of the battery's discharge capacity and voltage plateau.

[0006] To address the above problems, the present invention provides the following technical solution: In a first aspect, the present invention provides an ultra-low temperature resistant sodium-ion battery electrolyte, the electrolyte comprising the following components: An organic solvent, comprising propylene carbonate and methyl difluoroacetate, wherein the volume ratio of propylene carbonate to methyl difluoroacetate is 15:85 to 30:70. The main salt is sodium difluorosulfonamide, and its concentration in the electrolyte is 0.8 to 1.2 mol / L. The film-forming aid, based on 100% of the total mass of the electrolyte, comprises sodium difluorooxalate borate at a mass fraction of 1.0 wt% to 2.0 wt% and tris(trimethylsilyl) phosphite at a mass fraction of 0.2 wt% to 1.0 wt%. The acid removal additive, based on the total mass of the electrolyte (100%), is hexamethyldisilazane with a mass fraction of 0.05 wt% to 0.30 wt%. The electrolyte contains mesoscopic contact ion pair clusters, which are formed by the aggregation of the trimethylsilyl phosphite and sodium difluorooxalate borate in the solvation layer of sodium ions dissociated from the main salt through coordination and electrostatic interaction. These clusters are used to carry out a co-reduction reaction in situ on the negative electrode surface of the sodium-ion battery.

[0007] By employing the above technical solution, and by using a specific low-freezing-point solvent system to construct mesoscopic contact ion pair clusters in the liquid phase, the interfacial impedance under ultra-low temperature environments and the battery discharge performance under extremely cold conditions are reduced. The specific mechanism and reaction process are as follows: Step 1: Solvation coordination of free substances. Sodium difluorosulfonamide dissociates into free sodium ions in a mixture of propylene carbonate and methyl difluoroacetate. Tris(trimethylsilyl) phosphite and sodium difluorooxalate borate enter the solvation layer of sodium ions.

[0008] Step Two: Assembly of Mesoscopic Contact Ion Pair Clusters. In the liquid phase system, the lone pair electrons of the phosphorus atom in the trimethylsilyl phosphite molecule coordinate with sodium ions, while the anions dissociated from sodium difluorooxalate borate aggregate towards this center through electrostatic interactions, forming a cluster with the chemical formula [TMSPi-Na].+ -DFOB - The mesoscopic solvated cluster structure of ].

[0009] Step 3: Synchronous Co-reduction at the Anode Interface. Under ultra-low temperature conditions, conventional heterogeneous free monomeric film-forming materials cannot be effectively co-reduced due to the high activation energy of solvent desorption. In this scheme, the pre-assembled mesoscopic contact ion pair cluster structure diffuses as a whole to the anode interface. Since tris(trimethylsilyl) phosphite and sodium difluorooxalate borate are in adjacent coordination states on a spatial scale, they simultaneously accept electrons under the drive of the electrode potential, undergoing a co-reduction reaction.

[0010] Step 4: Formation of the cross-linked interfacial film. The products of the above co-reduction reaction undergo in-situ polymerization on the negative electrode surface, generating a network cross-linked structure containing boron-oxygen bonds, silicon-oxygen bonds, and phosphorus-oxygen bonds. This structure can resist the shrinkage stress of the battery electrode at ultra-low temperatures, prevent micro-crack failure of the interfacial film, and provide a low-impedance transport channel for sodium ion insertion / extraction.

[0011] Preferably, in the organic solvent, the volume ratio of propylene carbonate to methyl difluoroacetate is 20:80; the concentration of the main salt sodium difluorosulfonylimide in the electrolyte is 1.0 mol / L; the mass fraction of sodium difluorooxalate borate is 1.5 wt%; the mass fraction of tris(trimethylsilyl) phosphite is 0.5 wt%; and the mass fraction of hexamethyldisilazane is 0.15 wt%.

[0012] By adopting the above technical solution, propylene carbonate and methyl difluoroacetate at this specific volume ratio can balance the dissociation capacity of the system and the fluid viscosity at low temperature and high speed. The number of mesoscopic contact ion pairs clusters formed by each component in the liquid phase reaches a saturated state, avoiding the side reactions caused by excessive additives in the system, and further reducing the polarization resistance of the battery.

[0013] Preferably, the purity of each component raw material used to prepare the electrolyte is greater than or equal to 99.0%, and the moisture content of each component raw material is less than 10 ppm.

[0014] By adopting the above technical solution and strictly controlling the trace moisture content in the raw materials, it is possible to prevent water molecules from initiating the hydrolysis reaction of the main salt sodium difluorosulfonyl imide and the film-forming aid, thus ensuring the chemical stability of the cluster structure by the intermediate contact ions in the liquid phase.

[0015] Preferably, the deacidifying additive is used to neutralize free protons in the electrolyte to suppress solvation side reactions at the active sites of the trimethylsilyl phosphite.

[0016] By employing the above technical solution, the nitrogen atom in the hexamethyldisilazane molecule carries a lone pair of electrons, which can capture free hydrofluoric acid or protons generated by trace amounts of water in the electrolyte during the neutralization reaction. This process eliminates the acidic catalytic center in the heterogeneous system, prevents the degradation of the phosphorus atom active sites of tris(trimethylsilyl)phosphite under acidic conditions, and maintains the structural integrity of the film-forming precursor.

[0017] Secondly, the present invention provides a method for preparing an ultra-low temperature resistant sodium-ion battery electrolyte, comprising the following steps: S1. Under closed conditions, propylene carbonate and methyl difluoroacetate of the first volume are mixed and placed in a reaction vessel. The fluid is controlled at the first temperature and stirring is started. Premixed sodium difluorosulfonamide and sodium difluorooxalate borate are added and stirred until dissolved. S2. Adjust the temperature inside the reaction vessel to the second temperature, slowly add the remaining volume of methyl difluoroacetate, and circulate to mix and dilute; S3. Maintain the second temperature, inject protective gas into the liquid phase for submerged purging, and then add the hexamethyldisilazane and the tris(trimethylsilyl)phosphite in sequence and stir at a constant temperature. S4. Stop gas purging, seal the reaction vessel and fill it with protective gas to maintain pressure, raise the fluid temperature to the third temperature for isothermal ripening, and use thermodynamics to drive the film-forming aid to assemble the mesoscopic contact ion pair cluster structure. After the S5 and S4 steps of the curing process are completed, the fluid is immediately subjected to a rapid cooling treatment to reduce the core temperature of the fluid to the fourth temperature. Through the kinetic hysteresis effect, the mesoscopic contact ion pair cluster structure is frozen and retained in the system. Then, it is sealed and filled to obtain the ultra-low temperature resistant sodium ion battery electrolyte.

[0018] By employing the above technical solution, and through a specific process of heating and ripening followed by rapid quenching, a large-scale generation and stable preservation of mesoscopic contact ion pair cluster structures in a liquid phase system is achieved. The specific process mechanism and steps are as follows: The first stage involves constructing the reaction environment: mixing the main salt and solvent at relatively low first and second temperatures controls the heat of solution during the dissolution of the main salt, preventing localized overheating that could lead to solvent decomposition. During this stage, under-liquid purging with a protective gas and the addition of hexamethyldisilazane neutralize trace amounts of free protons within the system, providing an acid-free chemical environment for subsequent film-forming agents.

[0019] The second stage is thermodynamic ripening and assembly: the fluid is heated to a third temperature and maintained at a certain pressure, which provides thermodynamic energy to the film-forming agent molecules. Driven by this energy, tris(trimethylsilyl) phosphite and sodium difluorooxalate borate overcome the steric hindrance of methyl difluoroacetate molecules and the solvent desorption activation energy, enter the primary coordination loop of sodium ions, and assemble to form mesoscopic contact ion pair clusters.

[0020] The third stage is kinetic hysteresis freezing: immediately after ripening, a rapid cooling process is performed to lower the temperature to the fourth temperature. In the natural slow cooling process, the clusters formed inside the liquid phase undergo structural relaxation and disintegration, resulting in disordered secondary coordination. The rapid cooling process in this scheme blocks the kinetic process of cluster disintegration, utilizing the kinetic hysteresis of molecular motion to freeze and preserve the cluster configuration formed in the thermodynamic state at low temperature, ensuring the synchronous co-reduction capability of the film-forming precursor.

[0021] Preferably, in step S1, the first volume of methyl difluoroacetate accounts for 30% of the total volume of the formulation plan; the first temperature is 5°C to 15°C; the sodium difluorosulfonamide and sodium difluorooxalate borate are added in 2 to 4 batches, with an interval of 10 to 20 minutes between adjacent batches.

[0022] By adopting the above technical solution, the powder material is added in batches and dissolved within a temperature range of 5℃ to 15℃, controlling the exothermic rate during the dissolution process of sodium difluorosulfonyl imide and avoiding the ring-opening side reaction of propylene carbonate caused by instantaneous high temperature. Reserved methyl difluoroacetate for subsequent addition increases the polarity of the initial mixture and accelerates the dissolution rate of the powder material.

[0023] Preferably, in step S2, the second temperature is -10℃ to 0℃; in step S3, the protective gas is high-purity nitrogen, the underwater purging time is 20 to 40 minutes, after adding hexamethyldisilazane, the mixture is circulated and stirred at a constant temperature for 20 to 40 minutes, and then tris(trimethylsilyl) phosphite is added and the mixture is stirred for another 15 minutes.

[0024] By employing the above technical solution, the system temperature is lowered to below 0℃, reducing the saturated vapor pressure of the solvent. Combined with submerged nitrogen purging, trace amounts of dissolved oxygen in the system are carried out of the liquid phase. Hexamethyldisilazane is added first and then circulated and stirred to ensure complete neutralization of the free acid in the system before adding acid-sensitive tris(trimethylsilyl) phosphite to prevent degradation side reactions.

[0025] Preferably, in step S4, high-purity nitrogen is introduced and pressurized to 0.11 to 0.20 MPa, and the temperature of the fluid is increased at a rate of 1 to 3 °C / min; the third temperature is 35 °C to 45 °C, and the constant temperature curing time is 2 to 4 hours.

[0026] By employing the above technical solution, a slow heating rate of 1 to 3 °C / min ensures uniform heating of the liquid phase, avoiding convection caused by localized temperature differences that could interfere with the microscopic coordination process. The temperature range of 35 °C to 45 °C and the pressure environment of 0.11 to 0.20 MPa precisely overcome the activation energy barrier required for the film-forming aids to assemble into mesoscopic contact ion pair clusters. A ripening time of 2 to 4 hours ensures that the cluster assembly reaction reaches dynamic equilibrium.

[0027] Preferably, the rapid cooling process in step S5 is specifically implemented as follows: the fluid that has completed the curing process is immediately pumped into a tubular heat exchanger with a cooling medium temperature of -25°C to -15°C, the flow rate is controlled so that the residence time of the material in the heat exchanger is 1 to 2 minutes, and the center temperature of the fluid flowing out of the heat exchanger drops to 5°C to 9°C, which is the fourth temperature.

[0028] By adopting the above technical solution, the fluid undergoes a significant temperature drop within 1 to 2 minutes. The extremely high cooling rate causes stagnation of polarized molecular motion at the molecular level, preventing the relaxation and disintegration of the mesoscopic cluster structure during the cooling process, and realizing the trans-band retention of the high-temperature structural configuration to the room temperature and low-temperature states.

[0029] Preferably, the dew point of the sealed conditions is below -50°C.

[0030] By adopting the above technical solution, the dew point environment below -50℃ cuts off the pathway for external moisture to enter the reaction system, ensuring the chemical stability of the water-sensitive main salt and film-forming aid, and preventing the reaction of moisture with the main salt to generate hydrofluoric acid and initiate a chain of side reactions. In summary, the present invention has at least one of the following beneficial technical effects: 1. This invention reduces the macroscopic viscosity of the electrolyte at ultra-low temperatures by using a specific ratio of low freezing point solvent and main salt system, thus ensuring ionic conductivity, high discharge capacity retention, and a stable median voltage plateau under extremely cold conditions.

[0031] 2. The mesoscopic contact ion pair cluster structure pre-constructed in the liquid phase in this invention can diffuse as a whole to the negative electrode interface, overcoming the kinetic lag of conventional free monomers under extremely cold conditions and realizing synchronous co-reduction film formation with low energy barrier.

[0032] 3. The present invention generates a network cross-linked structure containing boron-oxygen, silicon-oxygen and phosphorus-oxygen bonds in situ through a cluster co-reduction reaction. This structure can effectively resist the destructive stress generated by electrode shrinkage under ultra-low temperature conditions and prevent the interface film from undergoing microcrack failure.

[0033] 4. The acid removal additive of this invention effectively neutralizes free protons caused by trace amounts of moisture in the system, eliminates acidic catalytic centers, and prevents acid-sensitive film-forming aids from undergoing degradation side reactions such as solvation.

[0034] 5. This invention employs a thermodynamic heating and ripening process combined with a kinetic rapid cooling process, which not only overcomes the energy barrier to complete cluster assembly, but also utilizes a cold-induced change to freeze the advantageous configuration, completely avoiding the structural relaxation and degradation caused by conventional slow cooling. Attached Figure Description

[0035] Figure 1 The following are comparative graphs of the physicochemical properties of the electrolytes in Examples 1 to 4 and Comparative Example 2 of the present invention, wherein (a) is a comparative graph of the free hydrofluoric acid content of each test object, and (b) is a comparative graph of the platinum-cobalt color of each test object. Figure 2 These are partial NMR spectra of Example 1, Comparative Examples 3 and 4 of the present invention, wherein (a) shows the film-forming additives in each test object. 31 P-NMR partial spectra, (b) showing the film-forming aids in each test object. 11 Partial B-NMR spectrum; Figure 3 The above are fitting curves of sodium ion desolvation activation energy for Example 1 and Comparative Examples 1, 5, and 6 of the present invention. Figure 4 The graph shows the long-cycle stability of Example 1, Comparative Example 1, and Comparative Example 3 at -40℃, where (a) is a curve showing the change in discharge capacity retention rate of each test object with the number of cycles, and (b) is a graph showing the evolution of coulombic efficiency of each test object. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0037] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0038] Sodium difluorosulfonamide (CAS No. 100669-96-3), propylene carbonate (CAS No. 108-32-7), methyl difluoroacetate (CAS No. 433-53-4), sodium difluorooxalate borate (CAS No. 2102517-30-4), tris(trimethylsilyl) phosphite (CAS No. 1795-31-9), and hexamethyldisilazane (CAS No. 999-97-3) used in the embodiments and comparative examples of this invention are all commercially available battery-grade products with a purity greater than or equal to 99.0%, and the moisture content is strictly controlled below 10 ppm.

[0039] Preparation Example 1: This preparation example provides a method for preparing a hard carbon negative electrode sheet, including the following steps: Active material hard carbon, conductive agent conductive carbon black (Super-P), and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 90:5:5, and then added to the solvent N-methylpyrrolidone (NMP). The mixture was stirred evenly in a vacuum mixer at 2000 rpm to prepare a negative electrode slurry with a solid content of 45%. The negative electrode slurry was uniformly coated onto a copper foil current collector with a thickness of 15 µm and baked in a forced-air drying oven at 80 °C for 12 h to remove the solvent. The dried electrode was then cold-pressed on a roller press to control the compaction density at 1.1 g / cm³. 3 The material was then cut into 14mm diameter round sheets using a punching machine and placed in a vacuum drying oven at 110℃ for 12 hours to obtain hard carbon negative electrode sheets.

[0040] Preparation Example 2: This preparation example provides a method for preparing a positive electrode sheet for a sodium-ion battery, including the following steps: The positive electrode active material is sodium copper iron manganate (NaCu). 0.1 Fe 0.2 Mn 0.7 O2), conductive carbon black (conductive agent), and polyvinylidene fluoride (PVDF) (binder) were mixed in a mass ratio of 90:5:5, and then added to N-methylpyrrolidone (N-methylpyrrolidone) solvent. The mixture was stirred evenly in a vacuum mixer at 2000 rpm to prepare a positive electrode slurry with a solid content of 60%. The positive electrode slurry was uniformly coated onto a 15µm thick aluminum foil current collector and baked in a forced-air drying oven at 80°C for 12 hours to remove the solvent. The dried electrode was then cold-pressed on a roller press to achieve a compaction density of 2.4 g / cm³. 3 The film was then cut into 14mm diameter round pieces using a punching machine and placed in a vacuum drying oven at 110℃ for 12 hours to obtain the positive electrode sheet for sodium-ion batteries.

[0041] Preparation Example 3: This preparation example provides a method for assembling a sodium-ion coin cell for testing, including the following steps: Assembly was performed in a glove box filled with high-purity argon gas, containing less than 0.1 ppm of water and less than 0.1 ppm of oxygen. The sodium-ion battery positive electrode obtained in Preparation Example 2 was used as the positive electrode, and the hard carbon negative electrode obtained in Preparation Example 1 was used as the negative electrode. A 16 µm thick polyethylene microporous membrane was used as the separator. The positive electrode, separator, and negative electrode were sequentially placed inside the positive electrode shell, and the electrolyte prepared in each example or comparative example was injected, with an injection volume of 80 µL. The negative electrode shell was then covered, and a sealing machine was used at 55 kg / cm². 2The cells are press-sealed under pressure. After assembly, the cells are left to stand at room temperature for 24 hours. Then, they are subjected to the first charge-discharge formation at a current rate of 0.05C on a battery testing system, with the voltage range controlled between 1.5V and 4.0V, to obtain sodium-ion coin cells for testing.

[0042] Example 1:

[0043] This embodiment provides a method for preparing an ultra-low temperature resistant sodium-ion battery electrolyte, including the following steps: (1) Under closed conditions with a dew point below -50°C, propylene carbonate (PC) and methyl difluoroacetate (MDFA) in the formula are mixed at a volume of 30% of the total planned addition amount and pumped into a jacketed reactor. The fluid temperature inside the reactor is controlled at 10°C and the mechanical stirring speed is turned on at 100 rpm. Sodium difluorosulfonamide (NaFSI) and sodium difluorooxalate borate (NaDFOB) powder are premixed and added in 3 batches with an interval of 15 min between adjacent batches. Stir until completely dissolved. (2) Adjust the temperature inside the reactor to -5℃, increase the stirring speed to 150rpm, slowly add the remaining methyl difluoroacetate (MDFA), and circulate and mix for 2h; the total volume ratio of PC to MDFA is 20:80. (3) Keep the temperature inside the reactor at -5℃, inject high-purity N2 into the liquid phase for microbubble purging for 30 min, add hexamethyldisilazane (HMDS) and circulate and stir at a constant temperature for 30 min, then add tris(trimethylsilyl) phosphite (TMSPi) and continue stirring for 15 min; (4) Stop the N2 purging, seal the reactor and fill it with high-purity N2 to maintain the pressure to 0.15 MPa. Raise the temperature of the fluid inside the reactor to 40°C at a heating rate of 2°C / min. Turn on the external circulation pump to maintain constant temperature curing for 3 hours. (5) After the curing process is completed, the liquid in the reactor is immediately pumped into a tubular heat exchanger with a cooling medium of -20℃. The flow rate is controlled so that the residence time of the material in the heat exchanger is 2 minutes, and the center temperature of the fluid flowing out of the heat exchanger drops to 5℃. Then, under the protection of N2 at 0.1MPa, it is pumped into a finished stainless steel storage tank for sealing and filling to obtain an ultra-low temperature resistant sodium-ion battery electrolyte. In the prepared electrolyte, the concentration of the main salt NaFSI is 1.0mol / L, the mass fraction of the film-forming aid NaDFOB is 1.5wt%, the mass fraction of TMSPi is 0.5wt%, and the mass fraction of the kinetic interceptor HMDS is 0.15wt%.

[0044] Example 2:

[0045] This embodiment provides a method for preparing an ultra-low temperature resistant sodium-ion battery electrolyte, including the following steps: (1) Under closed conditions with a dew point below -50°C, propylene carbonate (PC) and methyl difluoroacetate (MDFA) in the formula are mixed at a volume of 30% of the total planned addition amount and pumped into a jacketed reactor. The fluid temperature inside the reactor is controlled at 5°C and the mechanical stirring speed is turned on at 80 rpm. Sodium difluorosulfonamide (NaFSI) and sodium difluorooxalate borate (NaDFOB) powder are premixed and added in two batches with an interval of 10 min between adjacent batches. Stir until completely dissolved. (2) Adjust the temperature inside the reactor to -10℃, increase the stirring speed to 120rpm, slowly add the remaining methyl difluoroacetate (MDFA), and circulate and mix for 1h; the total volume ratio of PC to MDFA is 15:85. (3) Keep the temperature inside the reactor at -10℃, inject high-purity N2 into the liquid phase for microbubble purging for 20 min, add hexamethyldisilazane (HMDS) and circulate and stir at a constant temperature for 20 min, then add tris(trimethylsilyl) phosphite (TMSPi) and continue stirring for 15 min; (4) Stop the N2 purging, seal the reactor and fill it with high-purity N2 to maintain the pressure to 0.11 MPa, raise the temperature of the fluid inside the reactor to 35°C at a heating rate of 1°C / min, and turn on the external circulation pump to maintain constant temperature curing for 2 hours. (5) After the curing process is completed, the liquid in the reactor is immediately pumped into a tubular heat exchanger with a cooling medium of -25℃. The flow rate is controlled so that the residence time of the material in the heat exchanger is 1 minute, and the center temperature of the fluid flowing out of the heat exchanger drops to 5℃. Then, under the protection of N2 at 0.1MPa, it is pumped into a finished stainless steel storage tank for sealing and filling to obtain an ultra-low temperature resistant sodium-ion battery electrolyte. In the prepared electrolyte, the concentration of the main salt NaFSI is 0.8mol / L, the mass fraction of the film-forming aid NaDFOB is 1.0wt%, the mass fraction of TMSPi is 0.2wt%, and the mass fraction of the kinetic interceptor HMDS is 0.05wt%.

[0046] Example 3:

[0047] This embodiment provides a method for preparing an ultra-low temperature resistant sodium-ion battery electrolyte, including the following steps: (1) Under closed conditions with a dew point below -50°C, propylene carbonate (PC) and methyl difluoroacetate (MDFA) in the formula are mixed at a volume of 30% of the total planned addition amount and pumped into a jacketed reactor. The fluid temperature inside the reactor is controlled at 15°C and the mechanical stirring speed is turned on at 120 rpm. Sodium difluorosulfonamide (NaFSI) and sodium difluorooxalate borate (NaDFOB) powder are premixed and added in 4 batches with an interval of 20 min between adjacent batches. Stir until completely dissolved. (2) Adjust the temperature inside the reactor to 0°C, increase the stirring speed to 200 rpm, slowly add the remaining methyl difluoroacetate (MDFA), and circulate and mix for 3 hours to dilute; the total volume ratio of PC to MDFA is 30:70. (3) Keep the temperature inside the reactor at 0°C, inject high-purity N2 into the liquid phase for microbubble purging for 40 min, add hexamethyldisilazane (HMDS) and circulate and stir at a constant temperature for 40 min, then add tris(trimethylsilyl) phosphite (TMSPi) and continue stirring for 15 min. (4) Stop the N2 purging, seal the reactor and fill it with high-purity N2 to maintain the pressure to 0.20 MPa, raise the temperature of the fluid inside the reactor to 45°C at a heating rate of 3°C / min, and turn on the external circulation pump to maintain constant temperature curing for 4 hours. (5) After the curing process is completed, the liquid in the reactor is immediately pumped into a tubular heat exchanger with a cooling medium of -15℃. The flow rate is controlled so that the residence time of the material in the heat exchanger is 2 minutes, and the center temperature of the fluid flowing out of the heat exchanger drops to 8℃. Then, under the protection of N2 at 0.1MPa, it is pumped into a finished stainless steel storage tank for sealing and filling to obtain an ultra-low temperature resistant sodium-ion battery electrolyte. In the prepared electrolyte, the concentration of the main salt NaFSI is 1.2mol / L, the mass fraction of the film-forming aid NaDFOB is 2.0wt%, the mass fraction of TMSPi is 1.0wt%, and the mass fraction of the kinetic interceptor HMDS is 0.30wt%.

[0048] Example 4:

[0049] This embodiment provides a method for preparing an ultra-low temperature resistant sodium-ion battery electrolyte, including the following steps: (1) Under closed conditions with a dew point below -50°C, propylene carbonate (PC) and methyl difluoroacetate (MDFA) in the formula are mixed at a volume of 30% of the total planned addition amount and pumped into a jacketed reactor. The fluid temperature inside the reactor is controlled at 10°C and the mechanical stirring speed is turned on at 100 rpm. Sodium difluorosulfonamide (NaFSI) and sodium difluorooxalate borate (NaDFOB) powder are premixed and added in 3 batches with an interval of 15 min between adjacent batches. Stir until completely dissolved. (2) Adjust the temperature inside the reactor to -5℃, increase the stirring speed to 150rpm, slowly add the remaining methyl difluoroacetate (MDFA), and circulate and mix for 2h; the total volume ratio of PC to MDFA is 20:80. (3) Keep the temperature inside the reactor at -5℃, inject high-purity N2 into the liquid phase for microbubble purging for 30 min, add hexamethyldisilazane (HMDS) and circulate and stir at a constant temperature for 30 min, then add tris(trimethylsilyl) phosphite (TMSPi) and continue stirring for 15 min; (4) Stop the N2 purging, seal the reactor and fill it with high-purity N2 to maintain the pressure to 0.15 MPa. Raise the temperature of the fluid inside the reactor to 40°C at a heating rate of 2°C / min. Turn on the external circulation pump to maintain constant temperature curing for 3 hours. (5) After the curing process, the liquid in the reactor is pumped into a tubular heat exchanger with a cooling medium of -20℃. The flow rate is adjusted to shorten the residence time of the material in the heat exchanger to 1 minute, and the center temperature of the fluid flowing out of the heat exchanger drops to 9℃. Then, under the protection of N2 at 0.1MPa, it is pumped into a finished stainless steel storage tank for sealing and filling to obtain an ultra-low temperature resistant sodium-ion battery electrolyte. In the prepared electrolyte, the concentration of the main salt NaFSI is 1.0mol / L, the mass fraction of the film-forming aid NaDFOB is 1.5wt%, the mass fraction of TMSPi is 0.5wt%, and the mass fraction of the kinetic interceptor HMDS is 0.15wt%.

[0050] Comparative Example 1: Compared with Example 1, the difference is that hexamethyldisilazane (HMDS) is not added in step (3), and the thermodynamic heating and ripening in step (4) and the rapid cooling process in step (5) are not performed. The entire process is carried out at room temperature (25°C) using conventional physical mixing and stirring. All other aspects are the same.

[0051] Comparative Example 2: The difference from Example 1 is that hexamethyldisilazane (HMDS) is not added in step (3), but all other steps are the same.

[0052] Comparative Example 3: Compared with Example 1, the difference is that the thermodynamic heating and curing process in step (4) and the rapid cooling process in step (5) are not performed. After step (3) is completed, the temperature of the reactor is maintained at -5°C for 3 hours and then directly pumped into the finished stainless steel storage tank. The rest are the same.

[0053] Comparative Example 4: Compared with Example 1, the difference is that in step (5), after the curing process is completed, the reactor jacket heating system is shut down instead of being cooled by a tubular heat exchanger. The fluid in the reactor is allowed to cool slowly to 25°C at room temperature (takes more than 2 hours) before being pumped into the finished stainless steel storage tank. The rest are the same.

[0054] Comparative Example 5: Compared with Example 1, the difference is that the film-forming aid sodium difluorooxalate borate (NaDFOB) is not added in step (1), while the rest are the same.

[0055] Comparative Example 6: The difference from Example 1 is that trimethylsilyl phosphite (TMSPi) is not added in step (3), while the rest are the same.

[0056] Test Example 1: The testing steps are as follows: (1) In a glove box with an ambient dew point below -50°C, take 50 mL of each of the electrolyte samples prepared in Examples 1 to 4 and Comparative Example 2 and put them into sealed sampling bottles for later use.

[0057] (2) The free hydrofluoric acid (HF) content was determined using an automatic potentiometric titrator. Approximately 15 g of electrolyte sample was accurately weighed under an anhydrous environment and injected into a titration cup containing 50 mL of pre-cooled pure anhydrous methanol. Titration was performed using a 0.01 mol / L sodium methoxide standard titration solution under magnetic stirring. The volume of titrant consumed at the potential jump point was recorded. After three parallel determinations, the average value was taken and converted into the free hydrofluoric acid content (ppm) of the system.

[0058] (3) The color of the electrolyte was determined by a UV-Vis spectrophotometer combined with the platinum-cobalt colorimetric method. The electrolyte sample was transferred to a 10 mm quartz cuvette. Using ultrapure water as a blank reference, a full-band scan was performed in the wavelength range of 400 nm to 500 nm. The absorbance of specific absorption peaks was extracted and compared with the pre-prepared standard curve of the standard platinum-cobalt colorimetric ladder solution. The colorimetric value of the sample was directly read and recorded.

[0059] The test results are shown in Table 1.

[0060] Table 1. Results of free acidity and colorimetry tests of electrolytes in each example and comparative example:

[0061] According to Table 1 and Figure 1 According to the data, after the heating and aging process, the free hydrofluoric acid content of the electrolytes prepared in Examples 1-4 was controlled below 12 ppm, and the platinum-cobalt color value remained below 15 Hazen, indicating that the electrolyte was in a stable and transparent state. Sodium difluorosulfonamide usually retains trace amounts of moisture or protic acids during industrial preparation and purification. The film-forming additive tris(trimethylsilyl) phosphite in the system is sensitive to protic acids and is prone to ring-opening or solvation side reactions under acidic conditions. In Examples 1-4, hexamethyldisilazane was added before the heating and aging stage, utilizing the lone pair electrons on its nitrogen atom to preferentially neutralize the free protons in the electrolyte. This acid-base neutralization mechanism eliminates the acidic catalytic center in the system and inhibits the degradation of the sensitive additive during subsequent heating.

[0062] The test results of Comparative Example 2 showed that, without the addition of hexamethyldisilazane, direct heating and aging at 40°C significantly increased the free hydrofluoric acid content of the electrolyte to 156.4 ppm, increased the platinum-cobalt color value to 285 Hazen, and resulted in a solution appearance ranging from deep yellow to reddish-brown. The heating operation accelerated the hydrolysis of the main salt induced by trace amounts of moisture. The continuously generated free hydrofluoric acid triggered a side reaction at the P(III) site of tris(trimethylsilyl)phosphite, leading to irreversible polymerization and decomposition of the additive. These data indicate that, in the process of promoting contact ion pair assembly through heating and aging, the pre-addition of a free acid scavenger can avoid the hydrolysis of the main salt and the discoloration and failure of the additive caused by simple physical heating, and is a fundamental condition for ensuring the stable formation of the mesoscopic complex precursor.

[0063] Test Example 2: The testing steps are as follows: (1) In a glove box filled with high-purity argon gas where the oxygen content of the ambient water was below 0.1 ppm, 0.5 mL of the electrolyte samples from Example 1, Comparative Example 3, and Comparative Example 4 were each drawn using a micropipette and injected into a standard 5 mm NMR sample tube. A capillary tube sealed with deuterated chloroform (CDCl3) and a corresponding trace internal standard was inserted parallel to the tube as a coaxial reference to lock the magnetic field and eliminate signal interference caused by the macroscopic magnetic susceptibility of the solvent.

[0064] (2) A 400MHz liquid nuclear magnetic resonance spectrometer was used to acquire the spectra of the loaded sample tubes. The probe operating temperature was kept constant at 25℃, and the detection frequencies were adjusted to the corresponding resonance bands. 31 For P-NMR testing, the cumulative scan count was set to 128; for 11 For B-NMR testing, the number of scans was increased to 256 to ensure a sufficiently high signal-to-noise ratio.

[0065] (3) The collected free induction decay signal was subjected to Fourier transform and baseline calibration by the spectral processing system. The absolute chemical shift values ​​of the main peaks of phosphorus atoms in the trimethylsilyl phosphite molecule and boron atoms in the sodium difluorooxalate borate molecule were extracted, and the peak shift data caused by the change of electron cloud density were recorded.

[0066] The test results are shown in Table 2.

[0067] Table 2. Electrolytes of each embodiment and comparative example 31 P and 11 B-cell nuclear magnetic resonance chemical shift test results:

[0068] According to Table 2 and Figure 2The NMR spectroscopy data showed that the electrolyte system prepared in the example exhibited a significant low-field deshielding effect in the NMR test. When free molecules participate in coordination or form ion pairs, the peripheral electron clouds of the active center atoms are attracted by the central cation, resulting in a decrease in local electron cloud density and causing the chemical shift values ​​on the spectrum to shift towards higher frequencies or lower fields. The test results showed that the electrolyte system prepared in Example 1... 31 The chemical shift of the P-NMR main peak has shifted to 117.84 ppm. 11 The chemical shift of the main peak in the B-NMR spectrum is at 4.62 ppm. This synchronized chemical shift indicates that tris(trimethylsilyl) phosphite and difluorooxalate-borate anions overcome the electrostatic shielding of the solvent molecules. Under the thermodynamic driving force provided by the heating and ripening process, the two film-forming precursors enter the first solvation layer of sodium ions, forming a contact ion pair cluster structure in terms of spatial configuration.

[0069] In contrast, Comparative Example 3, which did not undergo a heating and curing process, showed that the main peak chemical shifts of phosphorus and boron atoms were in the high-field range of 114.31 ppm and 3.65 ppm, respectively. Mechanical mixing at room temperature is insufficient to break the solvated sheath layer formed between propylene carbonate and sodium ions. The film-forming precursor remains physically free due to the barrier effect of solvent molecules, which makes it difficult for the interfacial film-forming components to achieve simultaneous temporal and spatial reduction reactions during subsequent battery polarization. Furthermore, the data from Comparative Example 4 reflects the influence of the cooling method on the fluid microstructure. Although heating was performed during the preparation of this comparative example, the subsequent natural cooling at room temperature caused structural relaxation in the system. Since the contact ion pairs assembled by heating are metastable structures, during the slow temperature decrease, solvent molecules re-participate in coordination, causing the aggregated clusters to disintegrate and transform into solvent-separated ion pairs, thus manifesting as a regression in chemical shift values ​​in the spectrum. The rapid cooling process used in the embodiment blocks the degradation path of the structure. By rapidly cooling, the macroscopic viscosity of the system is increased in a short time. The kinetic hysteresis effect is used to stably retain the cluster structure evolved during the heating stage in the fluid base liquid at room temperature and low temperature, thus providing a structural basis for the construction of low activation energy ion channels in the battery under extreme conditions.

[0070] Test Example 3: The testing steps are as follows: (1) Take the coin cell assembled according to Preparation Example 3 and after the first formation, the electrolyte of which is the electrolyte sample prepared in Example 1 and Comparative Examples 1, 5 and 6 respectively. Each cell is subjected to 3 charge-discharge cycles at a current rate of 0.1C in a test chamber at a constant temperature of 25°C. Then, in the fourth cycle, it is charged to a full charge state of 4.0V, the polarization power supply is disconnected and it is left to stand for 4 hours to achieve internal electrochemical equilibrium.

[0071] (2) Transfer the fully charged battery to the high and low temperature alternating test chamber and connect the two electrodes to the test channel of the electrochemical workstation. Set the test temperature gradient to 25℃, 0℃, -20℃, -40℃ and -50℃. After each temperature point is reached, keep it at a constant temperature for 3 hours to ensure that the center temperature of the battery core or electrode is completely consistent with the ambient temperature.

[0072] (3) Apply a sinusoidal AC voltage signal with an amplitude of 5mV at each set temperature, and set the scanning frequency range to 100kHz to 0.01Hz to collect the corresponding electrochemical impedance spectra. Perform nonlinear least square fitting on the impedance spectrum using the built-in equivalent circuit model to extract the charge transfer impedance values ​​corresponding to the mid-to-high frequency semicircles.

[0073] (4) Based on the extracted charge transfer impedance values ​​at different temperatures, the reciprocal of the absolute temperature (1000 / T) is used as the horizontal axis, and the natural logarithm of the reciprocal of the impedance (ln(1 / R)) is used as the vertical axis. ct A scatter plot was plotted on the ordinate, and a linear regression was performed. The slope of the fitted line was extracted, and the activation energy of sodium ion desolvation for each battery system was calculated using the Arrhenius equation.

[0074] The test results are shown in Table 3.

[0075] Table 3. Test results of charge transfer impedance and desolvation activation energy of each battery at different temperatures:

[0076] According to Table 3 and Figure 3 According to the data, the battery system constructed with the electrolyte prepared in Example 1 exhibits low charge transfer impedance in the ultra-low temperature range. At room temperature, the interfacial impedance differences among the test groups are small; however, when the test temperature drops to -50℃, the limiting effect of different interfacial films on sodium ion transport increases significantly. At -50℃, the charge transfer impedance of Example 1 is 1284.62Ω, significantly lower than that of Comparative Example 1 (8952.37Ω) which uses a room-temperature mixed system. The fitting results of the desolvation activation energy show that the activation energy of Example 1 is 26.43 kJ / mol, indicating that the energy required for sodium ions to cross the solid-liquid interface is low. The reduction in electrochemical impedance is mainly attributed to the change in the mesoscopic structure of the electrolyte. Stable contact ion clusters in the electrolyte achieve simultaneous co-reduction reactions under the action of a polarized electric field, and tris(trimethylsilyl) phosphite and sodium difluorooxalate borate crosslink on the negative electrode surface to form an inorganic network structure containing POB bonds. The free volume provided by this structure alters the transport path of sodium ions as they detach from the surrounding solvent molecules, enabling ions to conduct through the inorganic network and thus reducing the transport resistance caused by the increased viscosity of the electrolyte in ultra-low temperature environments.

[0077] Comparative test results show that the systems in Comparative Examples 5 and 6 contain only a single film-forming additive (TMSPi or NaDFOB). Although this reduces the low-temperature impedance to some extent, their desolvation activation energy remains in the relatively high range of 38.12 kJ / mol to 39.85 kJ / mol. In polar solvent systems, the aforementioned monomer molecules are in a physically free state. Due to differences in their respective reduction potentials and diffusion coefficients, they tend to undergo layered deposition during the first charge of the battery. The interfacial layer composed of a single phosphorus- or boron-containing component lacks a three-dimensional cross-linked structure, making it difficult to simultaneously achieve high physical strength and ion conductivity. Furthermore, the data from Comparative Example 1 indicates that the electrolyte prepared by conventional mechanical mixing processes has a high desolvation barrier at low temperatures. The film formation process in this system depends on the in-situ reaction of each component on the electrode surface, resulting in a high proportion of organic components in the generated interfacial film, which is detrimental to the ion desolvation process at low temperatures. Tests show that by combining heating and curing with rapid cooling, the precursors for film formation can form a tightly packed complex in space, which can effectively reduce the charge transfer resistance at the solid-liquid interface and thus improve the ultra-low temperature kinetic performance of the battery.

[0078] Test Example 4: The testing steps are as follows: (1) In a standard constant temperature test chamber with an ambient temperature of 25°C, coin cells from Examples 1-4 and Comparative Examples 1-6, which had completed the formation process, were selected as experimental subjects and assembled according to Preparation Example 3. The cells were connected to the battery test system and charged to the upper limit voltage of 4.0V with a constant current of 0.2C. Then, the constant voltage charging stage was entered until the current decayed to 0.05C, so that it reached 100% full charge. At this temperature, a standard discharge was performed once to the cutoff voltage of 1.5V with a current rate of 0.1C, and the room temperature reference discharge capacity was recorded (the above discharge capacity was calculated based on the mass of the positive electrode active material). Then, the cells were fully charged again for use.

[0079] (2) Carefully transfer each group of batteries in full charge to the test fixture of the high and low temperature alternating test chamber, and connect the corresponding charge and discharge channel cables. Set the cooling program of the test chamber to cool down steadily to -50℃ at a rate of 1℃ / min, and keep it at a constant temperature for 8 hours after reaching the target temperature. This long-term deep cryogenic immersion process aims to ensure that the electrolyte in the porous electrode core and microporous membrane inside the battery reaches a state of extreme cold thermodynamic equilibrium, and eliminate the interference of internal and external temperature difference caused by the heat capacity of the shell.

[0080] (3) After confirming that the temperature inside the test chamber is stable at -50℃, the extreme cold discharge procedure is directly issued through the external test cabinet. The constant discharge current ratio is set to 0.1C, and the discharge cutoff voltage is kept constant at 1.5V. The test system automatically collects the terminal voltage and cumulative discharge capacity in real time, extracts the median voltage of the capacity integral of the entire discharge range, and calculates the extreme cold discharge capacity retention rate by dividing the total extreme cold discharge capacity by the normal temperature reference discharge capacity obtained in step (1).

[0081] The test results are shown in Table 4.

[0082] Table 4. Discharge performance test data of batteries from each embodiment and comparative example at -50℃ ultra-low temperature environment:

[0083] According to the data in Table 4, the electrolyte system prepared in the examples exhibited relatively stable mass transfer performance in the discharge test at -50℃. Polar carbonate solvents are close to their freezing point at -50℃, and the solidification of the solvent network usually leads to a significant decrease in bulk ionic conductivity and a substantial increase in interfacial impedance. The test results show that the discharge capacity retention rates of Examples 1-4 were all maintained above 70%, with Example 3 reaching 78.1%, and the median discharge voltage remained around 2.4V. These electrochemical performances indicate that the system alleviates the problem of prematurely reaching the cutoff voltage caused by severe ohmic polarization. From a microscopic mechanism analysis, the [TMSPi-Na] stably present in the electrolyte... + -DFOB - The contact ion pairs alter the film formation pathway at the solid-liquid interface. During the initial stages of charging and discharging, the aforementioned anion-cation complexes undergo in-situ co-reduction reactions on the negative electrode surface, constructing an inorganic network structure containing POB bonds. This structure provides mesoscale ion channels, allowing sodium ions to more easily escape the constraints of low-temperature, high-viscosity solvent molecules and conduct through high-dielectric inorganic channels, thereby maintaining the electrochemical reaction kinetics within the battery at ultra-low temperatures.

[0084] Comparative test results show that the conventional physical mixing system has weak low-temperature adaptability. Comparative Example 1's capacity retention dropped to 18.5% at -50℃, with a median voltage of 1.62V. This indicates that the film-forming material, free at room temperature, is difficult to effectively co-reduced due to kinetic lag at ultra-low temperatures. The battery electrode interface is mainly covered by by-reaction products of organic solvents, leading to a significant increase in interfacial impedance. The test data for Comparative Examples 3 and 4 reflect the influence of the preparation process on interfacial film formation. Comparative Example 3, without the heating and curing process, struggles to overcome the solvent desorption activation energy, and the film-forming precursor cannot effectively assemble into clusters. Comparative Example 4, which was heated but naturally cooled, experienced structural relaxation and disintegration of the clusters formed during the slow cooling phase. It not only lost its synchronous co-reduction capability but also formed a high-barrier configuration that is more difficult to dissociate than at room temperature due to disordered secondary solvation coordination. Therefore, the median discharge voltage of both examples is in the heavily polarized range of 1.8V to 2.0V, with Comparative Example 4 showing more significant capacity decay. Comparative Examples 5 and 6, when specific film-forming agents were added individually, lacked the cross-linking structural support of boron and phosphorus, resulting in insufficient physical strength of the generated single-component interfacial layer. Under the shrinkage stress of the low-temperature electrode, microcracks easily occurred, and their discharge capacity retention rate was only maintained at 40%–47%. Test data confirms that promoting the mesoscopic reconstruction of the film-forming precursor within the electrolyte through specific thermodynamic and kinetic processes is an effective means to improve the discharge performance of the battery under extremely cold conditions.

[0085] Test Example 5: The testing steps are as follows: (1) In a dry room where the ambient dew point is stably controlled below -50°C, coin cells that have undergone standardization were selected as experimental subjects and assembled according to Preparation Example 3. These cells were filled with electrolytes from Examples 1 to 4, Comparative Examples 1 and 3. The cells were then placed in batches on a dedicated multi-channel test rack in a high and low temperature alternating test chamber and connected to the fixture interface of a high-precision battery charge and discharge test system.

[0086] (2) The cooling program is set through the system host computer to reduce the temperature of the test chamber to -40℃ at a gradual rate of 0.5℃ / min, and then maintain the constant temperature at this target temperature for 12 hours. This long-range deep cryogenic immersion operation is crucial for eliminating the microscopic thermal gradient between the porous electrodes inside the battery and the external environment, ensuring that the subsequent electrochemical reaction is completely in extreme cold thermodynamic equilibrium.

[0087] (3) Start the long-cycle test under constant temperature conditions of -40℃. First, charge the battery with a constant current of 0.1C. When the terminal voltage reaches the upper limit of 4.0V, the system automatically switches to constant voltage charging mode until the charging current naturally decays to the cutoff threshold of 0.02C. Then, after 10 minutes of depolarization rest, discharge at the same rate of 0.1C to the lower cutoff voltage of 1.5V.

[0088] (4) The charging and discharging process in step (3) is set as a complete cycle, and a total of 100 cycles are continuously executed. The test system background will record the charging capacity, discharging capacity and median voltage of each cycle in real time, and use the discharge capacity at -40℃ for the first time as 100% as the benchmark to calculate the capacity retention rate for each cycle. At the same time, the ratio of the discharge capacity to the charging capacity of the same cycle is extracted to generate dynamic evolution data of coulombic efficiency.

[0089] The test data is shown in Table 5.

[0090] Table 5. Performance degradation data of each test object after 100 cycles of 0.1C / 0.1C at -40℃:

[0091] According to Table 5 and Figure 4 The data shows that the electrolyte system prepared in the examples exhibits good long-term interfacial stability under ultra-low temperature conditions of -40℃ and continuous charge / discharge at 0.1C / 0.1C. In the low-temperature environment, due to the reduced solid-phase ion diffusion rate, the polarization degree of the electrode surface increases, requiring the solid-liquid interface film (SEI) to withstand greater dynamic volume expansion and contraction stress. Test results show that after 100 cycles, the discharge capacity retention rate of Examples 1-4 remains at approximately 80%, with an average coulombic efficiency of approximately 99.85%. This electrochemical performance verifies the effectiveness of the mesoscopic precursor assembly mechanism. [TMSPi-Na] retained in the electrolyte... + -DFOB - During the initial formation stage, the ternary contact ion pairs underwent simultaneous dissociation and co-reduction, constructing a polymeric glass layer containing POB inorganic crosslinking bonds on the negative electrode surface. This structure not only provides a low-impedance ion transport channel, but its three-dimensional inorganic grid characteristics also endow the interfacial film with high mechanical toughness. This dense and relatively flexible protective layer effectively reduces the direct contact between polar solvent molecules and the sodium-intercalated negative electrode, suppressing electrode pulverization and structural damage caused by continuous solvent co-intercalation, thereby maintaining a stable microscopic interface morphology inside the battery under ultra-low temperature conditions.

[0092] Comparative test results show that the discharge capacity of Comparative Example 1 exhibits a significant decay trend. The capacity retention rate drops to 12.4% after 50 cycles, and the capacity is essentially depleted after 100 cycles. Its average coulombic efficiency of 98.21% reflects the continuous interfacial side reactions within the battery. Without the protection of film-forming additives, the interfacial film formed by a single carbonate solvent under a low-temperature polarized electric field is mainly composed of organic salts, with a relatively loose structure. Under cyclic alternating stress, it is prone to micro-cracks and damage, exposing fresh electrodes and triggering continuous electrolyte consumption, leading to irreversible loss of active sodium ions within the entire battery. The test results of Comparative Example 3 demonstrate the impact of the electrolyte preparation process on long-term cycle performance. Although phosphite and borate were added to this system, the lack of a heating and curing process made it difficult for the film-forming precursor to overcome the solvation energy barrier and form complex clusters. During battery charging and discharging, these free molecules undergo asynchronous reduction due to differences in redox potentials, resulting in an interfacial film that is a physical layering of phosphorus-containing and boron-containing components, lacking the support of a chemically cross-linked framework. In the early stages of cycling, the structure can maintain basic functions; however, with the accumulation of charge-discharge cycle stress, physical mismatch between different layers leads to the peeling of the film structure, which in turn causes an increase in cell impedance and irreversible capacity decay. Long-cycle test data further show that combining thermodynamic driving and kinetic freezing methods to promote the reconstruction of the electrolyte fluid structure helps to improve the interfacial structural stability and long cycle life of the battery under extremely cold conditions.

[0093] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An ultra-low temperature resistant sodium-ion battery electrolyte, characterized in that, The electrolyte contains the following components: An organic solvent, comprising propylene carbonate and methyl difluoroacetate, wherein the volume ratio of propylene carbonate to methyl difluoroacetate is 15:85 to 30:

70. The main salt is sodium difluorosulfonamide, and its concentration in the electrolyte is 0.8–1.2 mol / L. The film-forming aid, based on the total mass of the electrolyte (100%), comprises sodium difluorooxalate borate at a mass fraction of 1.0 wt% to 2.0 wt% and tris(trimethylsilyl) phosphite at a mass fraction of 0.2 wt% to 1.0 wt%. The acid removal additive, based on the total mass of the electrolyte (100%), is hexamethyldisilazane with a mass fraction of 0.05wt% to 0.30wt%. The electrolyte contains mesoscopic contact ion pair clusters, which are formed by the aggregation of the trimethylsilyl phosphite and sodium difluorooxalate borate in the solvation layer of sodium ions dissociated from the main salt through coordination and electrostatic interaction. These clusters are used to carry out a co-reduction reaction in situ on the negative electrode surface of the sodium-ion battery.

2. The ultra-low temperature resistant sodium-ion battery electrolyte according to claim 1, characterized in that, In the organic solvent, the volume ratio of propylene carbonate to methyl difluoroacetate is 20:

80. The concentration of the main salt, sodium difluorosulfonamide, in the electrolyte is 1.0 mol / L; The sodium difluorooxalate borate has a mass fraction of 1.5 wt%, the tris(trimethylsilyl) phosphite has a mass fraction of 0.5 wt%, and the hexamethyldisilazane has a mass fraction of 0.15 wt%.

3. The ultra-low temperature resistant sodium-ion battery electrolyte according to claim 1, characterized in that, The purity of each component raw material used to prepare the electrolyte is greater than or equal to 99.0%, and the moisture content of each component raw material is less than 10 ppm.

4. The ultra-low temperature resistant sodium-ion battery electrolyte according to claim 1, characterized in that, The acid-removing additive is used to neutralize free protons in the electrolyte, thereby inhibiting the solvolysis side reaction at the active sites of the trimethylsilyl phosphite.

5. A method for preparing an ultra-low temperature resistant sodium-ion battery electrolyte as described in any one of claims 1-4, characterized in that, Includes the following steps: S1. Under closed conditions, propylene carbonate and methyl difluoroacetate of the first volume are mixed and placed in a reaction vessel. The fluid is controlled at the first temperature and stirring is started. Premixed sodium difluorosulfonamide and sodium difluorooxalate borate are added and stirred until dissolved. S2. Adjust the temperature inside the reaction vessel to the second temperature, slowly add the remaining volume of methyl difluoroacetate, and circulate to mix and dilute; S3. Maintain the second temperature, inject protective gas into the liquid phase for submerged purging, and then add the hexamethyldisilazane and the tris(trimethylsilyl)phosphite in sequence and stir at a constant temperature. S4. Stop gas purging, seal the reaction vessel and fill it with protective gas to maintain pressure, raise the fluid temperature to the third temperature for isothermal ripening, and use thermodynamics to drive the film-forming aid to assemble the mesoscopic contact ion pair cluster structure. After the S5 and S4 steps of the curing process are completed, the fluid is immediately subjected to a rapid cooling treatment to reduce the core temperature of the fluid to the fourth temperature. Through the kinetic hysteresis effect, the mesoscopic contact ion pair cluster structure is frozen and retained in the system. Then, it is sealed and filled to obtain the ultra-low temperature resistant sodium ion battery electrolyte.

6. The preparation method according to claim 5, characterized in that, In step S1, the first volume of methyl difluoroacetate accounts for 30% of the total volume of the formula to be added; the first temperature is 5℃~15℃; the sodium difluorosulfonamide and sodium difluorooxalate borate are added in 2~4 batches, with an interval of 10~20min between adjacent batches.

7. The preparation method according to claim 5, characterized in that, In step S2, the second temperature is -10℃ to 0℃; in step S3, the protective gas is high-purity nitrogen, the liquid purging time is 20 to 40 minutes, after adding hexamethyldisilazane, the mixture is circulated and stirred at a constant temperature for 20 to 40 minutes, and then tris(trimethylsilyl) phosphite is added and the mixture is stirred for another 15 minutes.

8. The preparation method according to claim 5, characterized in that, In step S4, high-purity nitrogen gas is introduced and pressurized to 0.11–0.20 MPa, and the temperature of the fluid is increased at a rate of 1–3 °C / min; the third temperature is 35 °C–45 °C, and the constant temperature curing time is 2–4 h.

9. The preparation method according to claim 5, characterized in that, The specific method of the rapid cooling treatment in step S5 is as follows: the fluid that has completed the curing process is immediately pumped into a tubular heat exchanger with a cooling medium temperature of -25℃ to -15℃, the flow rate is controlled so that the residence time of the material in the heat exchanger is 1 to 2 minutes, and the center temperature of the fluid flowing out of the heat exchanger drops to 5℃ to 9℃, which is the fourth temperature.

10. The preparation method according to claim 5, characterized in that, The dew point of the sealed conditions is below -50°C.

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